You're correct that there's nothing in the Companion on this, and it is entirely possible Brandon came up with the Seed idea, but he does seem to have left the door open for a manufacturing heavy, highly technical second stage to making angreal.

And while Brandon may entirely have come up with the Seed himself, it does seem to have some basis in the series mythology. Assuming RJ wrote that the Finns can drain your OP strength (which I think is true, since Lanfear lost her ability, as did Moiraine, in the notes), there's already some basis for this. For another, the making of the Eye of the World also hints at spirit-y soul-y stuff, not advanced manufacturing. RJ even had Elayne feel a personal bond with her "wise woman" angreal, and speculate that it was a personal angreal.

So if it is a Brandon creation, it is a much more clever one than some other stuff he came up with for WoT.